The present invention relates to a telephone system including a central unit provided with an exchange service function and a plurality of digital key telephone sets, and more specifically to a telephone system including digital key telephone sets each connected to a data processing terminal unit such as a personal computer, for instance.
There is known a key telephone system such that a data processing terminal unit such as a personal computer is connectable to each extension telephone set to enable data communications between given extension data terminal units. The general system configuration of the above-mentioned key telephone system is as follows: a plurality of extension key telephone sets each provided with an interface unit (e.g. RS-232C) are connected to a key service unit provided with a switching unit for exchange service and a control unit, and each data processing terminal unit is connected to each interface unit (e.g. RS-232C) for communications with the key service unit.
In the telephone system as described above, the start, interruption, restart, end, etc. of data communications between the data terminal units are executed by the corresponding exchange service of the key service unit in response to commands given from each data terminal unit to request various data communication operations. For instance, in response to a command to request a call transmitted from a data terminal unit, a control unit of the key service unit controls a switching unit so as to connect a communication path between a calling data terminal unit and a called data terminal unit, so that data communications between the two data terminal units can be started. On the other hand, in response to a command to request an interrupt transmitted from one of the data terminal units now executing data communications, the key service unit disconnects the communication path. Thereafter, in response to a command to request a restart transmitted from the same terminal unit, the key service unit reconnects the same communication path to restart the data communications.
In the above-mentioned prior-art telephone system, there exists a problem in that it is impossible to immediately execute the interrupt or restart operation of data communications in response to commands transmitted from a data terminal unit.
In more detail, when an interrupt command is transmitted from one data terminal unit during data communications for some reasons, under the condition that the control unit of the key service unit is in an overload state (i.e. plural incoming and/or outgoing calls are generated simultaneously so that control processing is delayed momentarily), the communication path is disconnected with a delay after the command has been transmitted. As a result, communication data transmitted from one data terminal unit continue to be inputted to the other data terminal unit even after the interrupt command has been transmitted. In the case where the data terminal unit is connected to the interface unit (RS-232C) in three-wire connection method, in particular, the following problems arise:
In a three-wire connection system, data communications between the data terminal unit and the interface are effected through only two data lines (i.e. sending data (SD) line and receiving data (RD) line). In general, when receiving a control command transmitted from a data terminal unit, the interface unit transmits control data corresponding to the control command and thereafter returns a result code indicative of an end of control data transmission to the data terminal unit. In response to the returned result code, the data terminal unit confirms that the command transmitted from its own unit has been received normally. However, in case some communication data transmitted from one data terminal unit continue to be inputted to the other data terminal unit after the command has been transmitted as described above, since the communication data and the result code are transmitted via the same data (RD) line, the result code cannot be discriminated by the data terminal unit.
Further, where a command to request restart of interrupted data communications is transmitted by one data terminal unit, similar problems arise: when the control unit of the key service unit is in an overload state, since the communication path is reconnected with a delay after the restart command has been transmitted, the head portion of the communication data transmitted immediately after the restart command has been transmitted will not be received by the other data terminal unit.